Hybrid membrane-electrode assembly with minimal interfacial resistance and preparation method thereof

ABSTRACT

The present invention provides a membrane-electrode assembly comprising: electrodes consisting of a anode comprising a gas diffusion layer and a catalyst material-containing active layer, and an cathode comprising a diffusion layer and a catalyst material-containing active layer; and an electrolyte membrane interposed between the anode and the cathode and comprising a catalyst material-containing active layer at one or both sides, &#39;the electrodes being hot-pressed, to the electrolyte membrane, wherein in coating the active layer on the gas diffusion layer, the viscosity of the active layer is in a range of 100 to 10,000 cPs, as well as a production method thereof. The inventive membrane-electrode assembly has a low interfacial resistance between the membrane and the electrodes, as well as high catalyst availability and excellent power density, and can be mass-produced.

TECHNICAL FIELD

The present invention relates to a membrane-electrode assembly (MEA), akey component of a fuel cell, as well as a production method thereof.More particularly, the present invention relates to a membrane-electrodeassembly which is suitable for mass production and has a low interfacialresistance between the membrane and the electrode, as well as aproduction method thereof.

BACKGROUND ART

Fuel cells have recently received much attention as new electricgenerators. In the near future, the fuel cells will substitute for theexisting electric generators as either automobile batteries, powersources for electric generation or portable electric sources.

A polymer electrolyte fuel cell is a kind of a direct current generatorconverting the chemical energy of fuel directly to electric energy byelectrochemical reaction. It comprises a continuous stack complexequipped with membrane-electrode assemblies which are the heart of thefuel cell and bipolar plates which serves to collect generatedelectricity and to supply fuel. The membrane-electrode assembly refersto an assembly comprising: an electrode where electrochemical catalyticreaction occurs between fuel (aqueous methanol solution or hydrogen) andair; and a polymer membrane where the transfer of hydrogen ions occurs.

Meanwhile, all electrochemical reactions consist of two individualreactions, i.e., oxidation reaction occurring at an anode(fuelelectrode), and reduction reaction occurring at a cathode(airelectrode), in which the two electrodes are separated from each other bya polymer electrolyte membrane. In a direct methanol fuel cell, methanoland water in place of hydrogen are supplied to the anode, and hydrogenions produced in an oxidation process of methanol are transferred to thecathode through the polymer electrolyte membrane and generateselectricity by reduction reaction with oxygen supplied to the cathode.Such reactions are as follows:anode(fuel electrode): CH₃OH+H₂O→CO₂+6H⁺+6e ⁻cathode(air electrode): 3/2O₂+5H⁺+6e ⁻→30 3H₂OOverall reaction: CH₃OH+ 3/2O₂→CO₂+3H₂O

The electrode of the direct methanol fuel cell is typically a diffusionelectrode. The electrode consists of two layers, a gas diffusion layer(electrode support layer) and an active layer. The gas diffusion layerserves as a support and to diffuse fuel, and is made of carbon paper orcarbon cloth. The active layer is adjacent to the polymer electrolytemembrane to cause substantial electrochemical reaction and is made ofeither a platinum catalyst particle dispersed in a carbon particle orplatinum or alloy black. The electrochemical reaction occurs at athree-phase interfacial zone in which fuel diffused from the gasdiffusion layer is exposed to the interface between the electrolytemembrane and the platinum catalyst particle of the active layer. Thus,it is important for the improvement of performance to enlarge the areaof the three-phase reaction zone, which is available in theelectrochemical reaction, and to place the platinum catalyst in thethree-phase reaction zone to the maximum possible extent. However,unlike a liquid electrolyte, a depth to which the solid polymerelectrolyte membrane can be impregnated into the electrode is limited to10 μm, so that the area of the three-phase reaction zone, which can beenlarged, is limited, and only a portion of the platinum catalyst, whichis exposed to the three-phase reaction zone, can participate in theelectrochemical reaction in the fuel cell. Accordingly, in order toincrease the power density of the fuel cell, an electrode structure isrequired in which the area of the three-phase reaction zone is maximizedand the maximum possible amount of platinum is placed in the activelayer which is in contact with the electrolyte.

In the initial development stage of the direct methanol fuel cells, anelectrode was used which had been prepared by adding Pt-black particlesonto carbon paper or carbon cloth used as a diffusion layer by a spray,etc., so as to form an active layer, and adhering the active layer to anelectrolyte membrane by a hot-pressing process. However, the priorstructure had problems in that the interfacial resistance between theactive layer and the electrolyte membrane was high to make the structureinefficient, and a significant amount of the catalyst particlespenetrated into the diffusion layer and thus did not participate in thereaction, indicating that the expensive noble catalyst was useless.

In attempts to solve such problems, methods of forming a catalytic layerdirectly on an electrolyte membrane as in a decal process (U.S. Pat. No.6,391,486) and a sputter deposition process (U.S. Pat. No. 6,171,721)were proposed.

However, the decal process is one comprising forming an active layerseparately and then laminating the active layer with an electrolytemembrane, but requires a higher temperature than the glass transitiontemperature of the electrolyte membrane upon the laminating step, thusrequiring separate pretreatment which makes the process complex. Anotherproblem is that the transfer of the separately formed active layer isnot proferly performed.

In the sputter deposition process, the efficiency of a catalyst can beincreased, but a thin film is formed at a thickness of more than 1 μmdue to the crystalline nature of the catalyst, thus preventing thetransport of cations. Accordingly, only a very small amount of thecatalyst will inevitably be used, resulting in a reduction in powerdensity. Also, the sputter deposition process has problems in that ahigher power density than a given level can not obtained, and ahigh-vacuum region is used due to the characteristic of a semiconductorprocess, resulting in increases in production cost and time, whichrenders the process unsuitable for mass production.

While the above-described coating methods have their own advantages,they have a serious disadvantage in that it is difficult to form astable interface between a solid polymer electrolyte membrane andnanosized catalyst particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a hybrid membrane-electrodeassembly according to the present invention.

-   -   1: a diffusion layer of a anode;    -   2: an active layer of the anode;    -   3: an active layer of an electrolyte membrane;    -   4: an electrolyte membrane (polymer membrane);    -   5: an active layer of the electrolyte membrane;    -   6: an active layer of the cathode;    -   7: a diffusion layer of the cathode; and    -   8: a catalyst coated with electrolyte.

FIG. 2 shows the comparison of the laminating state of an electrolytemembrane to a catalytic active layer between a case where the catalyticlayer is formed on a diffusion layer (the upper portion of the figure)and a case where the catalytic layer is formed on an electrolytemembrane (the lower portion of the figure).

FIG. 3 shows microscope photographs of carbon cloth (left side) andcarbon cloth (right side), each of which is used as a diffusion layer.

FIG. 4 shows the participation of catalyst particles in reactionaccording to the viscosity of catalyst ink used in coating a diffusionlayer.

FIG. 5 is a schematic diagram showing a process of preparing catalystink.

FIG. 6 is a perspective view showing an electrolyte membrane which hadbeen coated with a catalytic active layer using a mask.

FIG. 7 is a schematic diagram showing a method of coating an activelayer on a gas diffusion layer by a screen printing process.

-   -   9: a screen printer;    -   10: an automatic feed head;    -   11: an active layer (catalytic layer);    -   12: a compression roller; and    -   13: a gas diffusion layer (carbon paper or carbon cloth).

FIG. 8 shows the current-voltage curve and power density curve ofmembrane-electrode assemblies produced in Comparative Examples 1 and 2and Example 1.

FIG. 9 shows not only schematic diagrams illustrating the coating stateof an active layer according to the viscosity of catalyst ink inelectrodes which had been produced by coating catalyst inks havingviscosities of 75 cPs (Comparative Example 3), 1,000 cPs (Example 1),and 15,000 cPs (Comparative Example 4), respectively, on a diffusionlayer by using a screen printing process, but also photographs of theelectrodes.

FIG. 10 shows the current-voltage curve and power density curve ofmembrane-electrode assemblies produced in Example 1 and ComparativeExamples 3 and 4.

FIG. 11 shows photographs of the front side (left figure) and backside(right figure) of an electrode where an active layer had been coated ata condition of 75 cPs by a die coating process in Comparative Example 5.

FIG. 12 shows the current-voltage curve and power density curve ofmembrane-electrode assemblies produced in Example 1 and ComparativeExample 6.

FIG. 13 shows photographs of an electrolyte membrane which had beencoated with an active layer by a screen printing technique inComparative Example 7.

FIG. 14 shows the structure of a catalytic layer coated on a gasdiffusion layer in membrane-electrode assemblies produced in Examples 2and 3.

FIG. 15 shows a current-voltage curve and power density curve inhydrogen fuel cells (PEMFC) which include membrane-electrode assembliesproduced in Examples 2 and 3, respectively.

DISCLOSURE OF THE INVENTION

Therefore, it is an object of the present invention to provide anepoch-making method which can solve the problem of formation of theunstable interface occurring in the prior art and allows catalystavailability to be increased.

The present inventors have extensive studies to produce amembrane-electrode assembly (MEA) overcoming the above-describeddisadvantages occurring in the prior art, and consequently found that ifa hybrid coating technique in which an active layer containing acatalyst material is coated on each of an electrolyte membrane and adiffusion layer forming interfacial resistance was used, interfacialresistance could be reduced as compared to that in a case of coating theactive layer on either of the electrolyte membrane and the diffusionlayer, and also the control of viscosity in forming the active layer onthe diffusion layer could result in an increase in catalystavailability. On the basis of these findings, the present invention hasbeen perfected.

Furthermore, the coating of the active layer on the gas diffusion layerby the hybrid coating technique was performed by a curtain coatingprocess, such as screen printing, die coating or blade coating, whichfacilitates mass production.

In one aspect, the present invention provides a membrane-electrodeassembly comprising: electrodes consisting of a anode comprising a gasdiffusion layer and a catalyst material-containing active layer, and ancathode comprising a diffusion layer and a catalyst material-containingactive layer; and an electrolyte membrane interposed between the anodeand the cathode and comprising a catalyst material-containing activelayer at one or both sides, the electrodes being hot-pressed to theelectrolyte membrane, wherein the viscosity of the active layer incoating the active layer on the gas diffusion layer is in a range of 100to 10,000 cPs (see FIG. 1).

In another aspect, the present invention provides a method for producinga membrane-electrode assembly, comprising the steps of: (a) forming acatalyst material-containing active layer on the surface of anelectrolyte membrane; (b) forming a catalyst material-containing activelayer on the surface of a gas diffusion layer; and (c) hot-pressing thegas diffusion layer to the electrolyte membrane, wherein the viscosityof the active layer, which is applied on the gas diffusion layer at thestep (b), is controlled in a range of 100 to 10,000 cPs. In this method,the steps (a) and (b) may be performed simultaneously, sequentially orin reverse order.

The present invention is characterized in that, in producing themembrane-electrode assembly, the catalytic active layer is coated oneach of the electrolyte membrane and the diffusion layer, and in formingthe active layer on the diffusion layer, the viscosity of the activelayer is controlled, in order to reduce interfacial resistance and toincrease catalyst availability and production rate.

(1) Reduction in Interfacial Resistance

The reaction in fuel cells mainly occurs at the interface between thecatalytic active layer and the electrolyte membrane. Namely, cationsgenerated from a catalyst are passed through the electrolyte membraneand subjected to electrochemical reaction at an opposite-side catalyst.For this reason, the laminating state between the electrolyte membraneand the catalytic active layer (formation of three-phase reaction zone)is very important.

As can be seen in FIG. 2, if the catalytic active layer is formeddirectly on the electrolyte membrane, a good laminating state betweenthe electrolyte membrane and the catalytic active layer will beobtained. On the other hand, coating a catalyst on the diffusion layerresults in resistance to the transport of cations generated from thecatalyst to the electrolyte membrane even if hot pressing is performed,since a depth to which the electrolyte membrane (nafion) penetrates intothe catalytic active layer is limited.

Interfacial resistance can be examined by impedance measurement. Forexample, the measurement of impedance of MEA in a single cell by atwo-electrode impedance method using a Zahner IM6 impedance analyzer,i.e., the measurement of impedance in a 1M-1 kHz region at analternating current amplitude of 5 mV using the flow of 400 sccmhydrogen gas through a reference electrode and the flow of 2000 sccm airthrough a working electrode, showed an interfacial resistance of 25-30mΩ*6.25 for the direct coating (the case of forming a catalytic layer onan electrolyte membrane), and an interfacial resistance of 35-40 mQ*6.25for the indirect coating (the case of forming a catalytic layer on adiffusion layer).

Thus, the present invention aims to reduce interfacial resistance bycoating the catalytic active layer on both the electrolyte membrane andthe gas diffusion layer in the membrane-electrode assembly.

(2) Increase in Catalyst Availability

A first approach to increase catalyst availability is to increase theelectrochemical utilization efficiency of the catalyst in MEA, and asecond approach is to increase the amount of the catalyst coated duringthe production of MEA.

(a) The electrochemical reaction in a fuel cell occurs in a three-phasereaction zone where a catalyst (e.g., Pt or Pt—Ru), fuel (e.g., methanolsolution), and ionomer (e.g., nafion ionomer) are present together.Thus, a catalyst which is not present in the three-phase reaction zonedoes not participate in the reaction and causes a reduction in catalyticefficiency.

If dilute catalyst ink is used in forming the catalytic layer on the gasdiffusion layer (GDL), the catalyst particles with a size of less thanabout 1 μm will penetrate into the pores of the diffusion layer (seeFIGS. 3 and 11), thus reducing the amount of the catalyst used in thereaction. In order to increase the catalytic efficiency (i.e., theefficiency of MEA in the electrochemical reaction), the catalystparticles must not be lost into the diffusion layer (GDL) (see FIG. 4).

For this purpose, the present invention is characterized in that theionomer membrane (Nafion membrane), an electrolyte membrane where themain reaction occurs, is coated with the catalytic active layer by anair spray process, and catalyst ink with high viscosity is coated on thegas diffusion layer (GDL) by screen coating or die coating. This allowsthe fabrication of the catalytic active layer without the loss of thecatalyst into the diffusion layer.

(b) In the air spray process used in coating the catalytic layer on theelectrolyte membrane, the sprayed catalyst particles shows a very lowadhesion rate of about 30 wt % and a loss rate of the remaining 70%,resulting in a reduction in catalyst availability. Thus, the presentinvention is characterized in that a small amount of the catalytic layeris coated on the electrolyte membrane in order to reduce the interfacialresistance of the electrode, and most of the catalytic active layer iscoated on the diffusion layer (an adhesion rate of more than 90% for arotary screen printing process) in order to increase the catalystavailability.

(3) Increase in Production Rate

In coating most of the catalytic active layer on the diffusion layer,the present invention utilizes a curtain coating process, such as screenprinting, die coating or blade coating, which makes mass productioneasy.

Hereinafter, the present invention will be described in detail.

1. Preparation of Catalyst Ink for Active Layer

Both an anode (fuel electrode) and a cathode (air electrode) in a fuelcell contain a catalytic material for electrochemical reaction in anactive layer.

Catalyst materials which can be used in the present invention include Ptblack, Pt—Ru black, platinum-supported carbon (platinized carbon, Pt/C),platinum-ruthenium-supported carbon (Pt—Ru/C), platinum-molybdenum(Pt—Mo) black, platinum-molybdenum (Pt—Mo)-supported carbon,platinum-rhodium (Pt—Rh) black, platinum-rhodium-supported carbon, andother platinum based alloys.

The catalyst for the anode is preferably Pt—Ru or Pt—Ru/C, and thecatalyst for the cathode is preferably Pt or Pt/C. The reaction in theanode generates CO which poisons the catalyst, thus lowering thecatalyst activity. In order to prevent this phenomenon, Ru is preferablyused as a cocatalyst.

The catalyst particles may be dispersed in carbon particles or made ofplatinum or alloy blacks.

Examples of catalyst supports which can be used in the present inventioninclude all general carbon black based supports such as Vucan XC-72R,Vulcan XC-72, acetylene black, Kejon black, and black pearl, as well asconducting complex oxides such as platinum oxide and ruthenium oxide.

The catalyst material of active layer applied on the surface of theelectrolyte membrane, which is opposite to the surface of the anode, ispreferably the same as the active layer material on the anode, and thecatalyst material of active layer applied on the surface of theelectrolyte membrane, which is opposite to the surface of the cathode,is preferably the same as the active layer material on the cathode.

However, the composition and coating method of the catalyst ink appliedon each of the diffusion layer and the electrolyte membrane aredifferent between the diffusion layer and the electrolyte membrane. Inother words, the electrolyte membrane is coated using a catalyst inkhaving a low viscosity of less than 10 cPs as it is preferably coated byan air spray process. Also, the gas diffusion layer (GDL) is preferablycoated using a catalyst ink having a high viscosity of 100-10,000 cPsand more preferably 1,000 cPs, so that it is preferably coated by ascreen printing, blade coating or die coating process.

A solvent/dispersion medium for the active layer of the anode ispreferably the same as a solvent/dispersion medium for the active layerof the cathode. Non-limited examples of the solvent/dispersion medium,which can be used in the present invention, include water, butanol,isopropanol (IPA), methanol, ethanol, normal propanol, normal butylacetate, and ethylene glycol.

The content of the solvent/dispersion medium in the catalyst ink ispreferably 1-30% times the weight of the catalyst used. The viscosity ofthe catalyst ink may vary depending on the amount of the catalyst, and acoating process may be determined depending on the viscosity.

In order to increase the utilization efficiency of the catalyst, it isimportant to make catalyst ink where the catalyst is well dispersedwithout aggregation. For this purpose, it is preferable in the presentinvention that isopropanol (IPA), NAFION solution and water are mixedwith each other at suitable amounts to prepare a well-dispersed solventmixture, which is mixed and stirred with the catalyst so as to dispersethe catalyst well and then subjected to a ultrasonic milling process for5 minutes so as to be uniformly mixed with the catalyst. On apreparation method of the catalyst ink, see FIG. 5.

The composition of the catalyst ink for the active layer of the anodecontains, but not limited to, Pt—Ru or Pt—Ru/C, Nafion ionomer (30 wt %based on the weight of the catalyst), and solvent (1-30 times the weightof the catalyst), and the composition of the catalyst ink for the activelayer of the cathode contains, but not limited to, Pt or Pt/C, Nafionionomer (30 wt % based on the weight of the catalyst), and solvent (1-30times the weight of the catalyst)

2. Coating of Catalyst Ink on Electrolyte Membrane (First Step)

The electrolyte membrane acts as a hydrogen ion (H⁺) conductor.

Non-limited examples of the electrolyte membrane, which can be used inthe present invention, include Nafion™ membrane (manufactured by DuPontCorp, perfluoro sulfonic acid), Flemion (Asahi glass Co.), aciplex(Asahi Chemical Co.), and Gore-select (Gore Co.), as well as all cationelectrolyte membranes.

The electrolyte membrane may be either a complex electrolyte membrane ora membrane, the surface of which had been hydrophilically treated.

Coating the electrolyte membrane with catalyst ink will be preferablyperformed by supplying the catalyst ink by a gas pressure method andcoating the catalyst ink on a completely dried electrolyte membrane by aspray process. In this case, the viscosity of the active layer which iscoated on the electrolyte membrane is preferably in a range of 1-10 cps.

In the first step, the coating of the catalyst ink on the surface of theelectrolyte membrane is preferably performed by a spray coating process.This is because the spray coating process has an advantage in that itallows a thin catalyst layer to be formed directly on the surface of thepolymer electrolyte membrane, thus securing a continuous interface andincreasing the availability of the catalyst. Another reason is thatcoating the catalytic active layer on the electrolyte membrane by ascreen method results in the deformation of the electrolyte membrane(see FIG. 13).

At this time, in order to prevent the electrolyte membrane from beingswollen by the solvent of the catalyst ink during the production of themembrane-electrode assembly, it is important to maintain the electrolytemembrane in a dried state. It is thus preferable to continuouslyevaporate the catalyst ink solvent by a thermal dryer during the spraycoating, so as to maintain the electrolyte membrane in a dried state.For this purpose, an air spray process is preferably used.

An example of the spray coating process uses a spray gun.

Examples of carrier gas which can be used in the spray coating using thespray gun include inert gases, such as nitrogen and air. The pressure ofthe carrier gas in the spray coating using the spray gun may be in arange of 0.01 to 2 atm.

An operation temperature at which the active layer is formed on theelectrolyte membrane is preferably in a range of 20 to 100° C. Anoperation temperature of more than 120° C. may cause the problem ofNafion degradation as it approaches 140° C., the glass transitiontemperature (Tg) of Nafion. Furthermore, the operation temperature mayvary depending on the Tg of an electrolyte membrane used.

In addition to the spray process, processes of coating the catalyst inkon the electrolyte membrane include a physical vapor deposition processof coating a very small amount of a catalyst on a membrane using a RFmagnetron sputter or a thermal evaporator. Particularly, in order toprevent the crystal growth of the catalyst upon sputtering,co-sputtering with carbon or conductive material may be adopted.

The electrolyte membrane is preferably coated with the active layerusing a mask. Namely, it is preferable to form patterns. This is becausethe electrolyte membrane needs to have a larger area than an arearequired for reaction since it must not only act as a passage for thetransport of an electrolyte but also act to inhibit the flow ofreactants (methanol or hydrogen, oxygen) (see FIG. 6).

The amount of the active layer formed on the electrolyte membrane ispreferably 1-100% by weight based on the weight of the active layerformed on the diffusion layer. The active layer on the electrolytemembrane is preferably formed to a thickness of less than 1 μm. This isbecause it is advantageous to coat most of the catalyst on the diffusionlayer since a coating process such as screen printing is moreadvantageous for mass production than that of the spray process.

3. Coating of Catalyst Ink on Diffusion Layer (Second Step)

Although catalyst ink used on the diffusion layer is made of the samematerial as that of the catalyst ink used on the electrolyte membrane,it preferably contains a solvent at a different ratio from that of thecatalyst ink used on the electrolyte membrane. Specifically, it ispreferable that the catalyst ink on the diffusion layer has thickviscosity, and the catalyst ink on the electrolyte membrane has diluteviscosity.

The diffusion layer may be formed of carbon paper or carbon cloth(manufactured by SGL, toray, etc.).

If methanol is used as fuel, it will be preferable that the diffusionlayer used in the anode is made of a carbon paper or carbon cloth whichhad not been water-repellent-coated with Teflon in order to effectivelysupply methanol. In this case, the diffusion layer of the cathode ispreferably made of a carbon paper which had been water-repellent-coatedwith 5-20 wt % of Teflon in order to remove water generated afterreaction.

The catalyst ink used in forming the active layer on the gas diffusionlayer preferably has a viscosity of 100-10,000 cPs, and more preferably1,000 cPs.

In the process of forming the active layer on the diffusion layer, it ismost important to control the viscosity of the catalyst ink. This isbecause the current-voltage curve and power density of themembrane-electrode assembly vary depending on the control of viscosityin forming the active layer on the diffusion layer. Namely, if thecatalyst ink has low viscosity, catalyst particles will infiltrate intothe porous diffusion layer, so that catalyst particles which do notparticipate in reaction will significantly increase so as to makecatalyst performance insufficient and to require the use of a largeamount of catalysts. Particularly, the infiltrated catalyst particlesact as serious resistance to the diffusion of the fuel methanol solutionat the diffusion layer side of the anode. For this reason, catalyst inkwith high viscosity is required.

Processes of coating the catalyst ink for the active layer on thediffusion layer include a curtain coating process (e.g., screenprinting, blade coating or die coating) and a spray process.

In order to prevent catalyst particles (less than 1 μm) frominfiltrating into the internal pores of the diffusion layer to lowercatalyst availability, catalyst ink with the highest possible viscosityneeds to be used. Thus, the curtain coating process is more advantageousthan a spray process such as air spray. Further, the curtain coatingprocess is advantageous for mass production

In forming a catalytic layer on the gas diffusion layer, a catalystcoated with electrolyte powder is preferably used in order to form athree-phase reaction zone while facilitating the control of the catalystink viscosity. Then, the electrolyte (Nafion)-coated catalyst powder ispreferably mixed with a solvent or dispersion medium. Examples of thesolvent or dispersion medium used include water and alcohols, such asbutanol, isopropanol (IPA), and normal propanol (NPA). If the Nafionsolution is added to the catalyst ink so as to form the three-phasereaction zone, the viscosity of the catalyst ink will be reduced to makethe catalyst ink unsuitable for screen printing. Thus, it is preferableto control the catalyst viscosity by performing a pretreatment processwhere a Pt or Pt—Ru black catalyst is added to Nafion solution, and thesolvent of the Nafion solution is dried in a drying oven, thus coatingonly Nafion electrolyte on the surface of the catalyst particles. By thecatalyst pretreatment process as described above, the catalyst viscositymay be easily determined depending on the mixing ratio of the catalystto the solvent.

A process of coating the catalyst ink prepared as described above on thegas diffusion layer using a screen printer is shown in FIG. 7.

As a coating process such as screen printing is more advantageous formass production than the spray process, it is advantageous to coat mostof the catalyst on the diffusion layer. Thus, it is preferable to formthe active layer on the electrolyte membrane to a thickness of less than1 μm while forming most of the active layer on the diffusion layer.

In the step of forming the active layer on the gas diffusion layer, itis preferable to dry the solvent of the catalyst ink remaining aftercoating the catalytic layer. The active layer formed on the gasdiffusion layer is preferably dried by a hot rolling process, thusmaking an electrode.

4. Assembly of electrode and electrolyte membrane The active layercoated on the electrolyte membrane adheres to the active layer(catalytic layer) coated on the gas diffusion layer by hot pressing,thus producing a membrane-electrode membrane (MEA).

The process of hot-pressing the electrode to the electrolyte membrane ispreferably performed at a temperature of 50-200° C. under a pressure of5-100 kg/cm².

The Advantageous Effect

The inventive membrane-electrode assembly has a low interfacialresistance between the membrane and the electrodes, as well as highcatalyst availability and excellent power density, and can bemass-produced.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail byexamples. It is to be understood, however, that these examples are givento illustrate the present invention and not intended to limit the scopeof the present invention.

Example 1

Step 1: Coating on Electrolyte Membrane Side

Pt—Ru black was used as a catalyst on a anode side of an electrolytemembrane, and Pt black was used as a catalyst on an cathode side of theelectrolyte membrane. Nafion solution, IPA and water were mixed witheach other at suitable amounts so as to prepare a well-dispersed solventmixture. The solvent mixture was mixed with each of the catalysts at aratio of catalyst:Nafion dry weight:dispersion medium of 1:0.3:50, andstirred to disperse the catalyst well, and uniformly mixed bysonification for 5 minutes, thus preparing catalyst ink. The catalystink had a viscosity of 1 cPs.

A Dupont Nafion membrane used as an electrolyte membrane was pretreatedwith hydrogen peroxide and sulfuric acid and then inserted into astainless grid used as a mask. The Nafion membrane was heated at thebackside of the stainless grid by a thermal dryer for 20 minutes so asto completely remove moisture. The temperature in the thermal dryer was80° C. Next, each of the catalyst inks (Pt—Ru black for the anode side,and Pt black for the cathode side) prepared as described above was takenand sprayed on the front side of the grid at the amount of 0.1-20 cc/cm²by means of a spray gun, thus forming an active layer with a thicknessof less than 1 μm. At this time, the pressure of carrier gas was in arange of 0.01-2 atm, and the solvent of the catalyst ink wascontinuously evaporated during the spray coating by heating the Nafionmembrane at the backside of the grid by a thermal dryer.

Step 2: Coating on Diffusion Layer

As a diffusion layer for use on the anode, carbon paper and carbon clothwhich had not been water-repellent-coated with Teflon in order toefficiently supply methanol was used. As a diffusion layer for use inthe cathode, carbon paper which had been water-repellent-coated with5-20 wt % of Teflon in order to remove water generated after reactionwas used.

A pretreatment process was performed in which each of Pt black and Pt—Rublack catalysts was added to Nafion solution, and the solvent of theNafion solution was dried in a drying oven, thus coating only Nafionelectrolyte on the surface of the catalyst particles.

As the anode catalyst, the Pt—Ru black pretreated as described above wasused, and as the cathode catalyst, the Pt black pretreated as describedabove was used. IPA and water were mixed with each other at suitableamounts so as to prepare a well-dispersed solvent mixture, and thenmixed with each of the catalysts at a ratio of catalyst:naf ion dryweight:dispersion medium of 1:0.3:3. The resulting mixture was stirredto disperse the catalyst well, and uniformly mixed by sonication for 5minutes, thus preparing catalyst inks. The catalyst inks had a viscosityof 1000 cPs.

Each of the catalyst inks prepared as described above was coated on adiffusion layer by means of a screen printer shown in FIG. 7, thusmaking an electrode with a catalyst content of about 4 mg/cm². Theactive layer formed on the diffusion layer was dried by a hot rollingprocess, thus producing a diffusion electrode.

Step 3: Assembling of Electrode and Electrolyte Membrane

A Nafion electrolyte membrane containing an active layer was put betweenthe two electrodes prepared as described above, and hot-pressed at 140°C. under a pressure of 5-100 kg/cm² for 3-10 minutes, thus producing amembrane-electrode assembly (MEA). In this MEA, the Nafion electrolytemembrane was slightly larger than the electrodes, and the sizes of theelectrodes and the Nafion electrolyte membrane were 5cm² and 16 cm²,respectively.

Comparative Example 1 Direct Coating

Example 1 was repeated except that, in the preparation of MEA, thecatalytic layer (active layer) was formed directly only on theelectrolyte membrane using catalyst ink having a viscosity of 1 cPs byan air spray process.

Comparative Example 2 Indirect Coating

Example 1 was repeated except that, in the preparation of MEA, thecatalytic layer (active layer) was coated only on the diffusion layerusing catalyst ink having a viscosity of 1 cPs by an air spray processand then assembled with the electrolyte membrane by a hot pressingprocess.

Comparative Example 3 Coating of Diffusion Layer with Catalyst InkHaving Viscosity of Less than 100 cPs

Example 1 was repeated except that catalyst ink with a viscosity of 75cPs was used in coating the catalytic layer (active layer) on thediffusion layer.

Comparative Example 4 Coating of Diffusion Layer with Catalyst InkHaving Viscosity of More than 10,000 cPs

Example 1 was repeated except that catalyst ink with a viscosity of15,000 cPs was used in coating the catalytic layer (active layer) on thediffusion layer.

Comparative Example 5 Die Coating of Diffusion Layer with Catalyst InkHaving Viscosity of Less than 100 cPs)

Example 1 was repeated except that the catalytic layer (active layer)was coated on the diffusion layer using catalyst ink having a viscosityof 75 cPs by a die coating process.

FIG. 11 shows photographs of the front side (left figure) and backside(right figure) of the electrode prepared according to ComparativeExample 5.

As shown in FIG. 11, it can be found that if catalyst ink with aviscosity of less than 100 cPs is used, catalyst particles willpenetrate the electrode support (diffusion layer) so as to impregnatethe diffusion layer to the backside of the diffusion layer. Theimpregnation of the diffusion layer with the catalyst particles causesan increase in the mass transfer resistance of fuel, thus showing anegative effect on performance.

Comparative Example 6 Coating of Catalyst Ink on Diffusion Layer bySpray Coating Process

Example 1 was repeated except that the catalytic layer (active layer)was coated on the diffusion layer using catalyst ink having a viscosityof 1 cPs by a spray coating process. Namely, the active layer was coatedon both the diffusion layer and the electrolyte membrane by a spraycoating process.

Comparative Example 7 Coating of Catalyst Ink on Electrolyte Membrane byScreen Printing

Example 1 was repeated except that the catalytic layer (active layer)was coated on the electrolyte membrane using catalyst ink having aviscosity of 1000 cPs by a screen printing process.

As shown in FIG. 13, when the active layer was coated on the electrolytemembrane by the screen printing process, one of mass productionprocesses, the Nafion membrane used as the electrolyte membrane would beseverely deformed. Due to the severe deformation of the electrolytemembrane, the catalyst particles were not uniformly coated to make theproduction of MEA impossible.

Example 2

Example 1 was repeated except that a Pt/C (Pt on Carbon) in place ofeach of Pt—Ru black and Pt black was used as a catalyst (see FIG. 14)

Example 3 Use of Catalyst Particles Uncoated with Nafion

Example 2 was repeated except that, in the preparation of catalyst inkto be coated on the diffusion layer, catalyst particles non-pretreatedwith the Nafion solution were mixed with a solvent mixture of IPA,Nafion solution and water to prepare the catalyst ink (see FIG. 14).

Experiments

1. Measurement of Interfacial Resistance (Example 1 and ComparativeExample 2)

The conductivity of MEA in a single cell was measured with a Zahner IM6analyzer by a two-electrode impedance method. 400 sccm of hydrogen gaswas diffused through a reference electrode, 2000 sccm of air wasdiffused through a working electrode, and the impedance in a 1M-1 kHzregion was measured at an alternating current amplitude of 5 mV.

The membrane-electrode assembly of Comparative Example 2 (indirectcoating) had an interfacial resistance of 35-40 mQ*6.25, whereas themembrane-electrode assembly produced by separately coating the activelayer according to Example 1 (direct coating) had a low interfacialresistance of 25-30 mΩ2*6.25.

2. Measurement of Power Density (Example 1 and Comparative Examples 1and 2)

The power densities of the membrane-electrode assemblies produced inExample 1 and Comparative Examples 1 and 2 were measured.

The power density measurement was carried out under the followingoperation conditions of a single cell: anode catalyst: Pt/Ru black;cathode catalyst: Pt black; operation temperature: 80° C.; amount of useof catalyst: 4 mg/cm²; fuel: 2M CH₃OH; 1000 cc/min of oxygen; andambient pressure.

FIG. 8 shows power density curves according to the production methods ofthe membrane-electrode assembly. As shown in FIG. 8, it can be foundthat the use of the hybrid coating method according to Example 1 shows amore than 50% increase in power density.

Namely, it can be found from FIG. 8 that even the use of the catalystswith the same amount shows the deviation in performance, and theindirect-direct hybrid coating method has excellent performance. Thissuggests that the membrane-electrode assembly produced according to thepresent invention has high catalyst availability.

3. Measurement of Coating State and Power Density According to Viscosityin Screen Coating on Diffusion Layer (Example 1 and Comparative Examples3 and 4)

FIG. 9 shows not only schematic diagrams illustrating the coating stateof the active layer according to the viscosity of catalyst ink in theelectrodes which had been produced by coating catalyst inks havingviscosities of 75 cPs (Comparative Example 3), 1,000 cPs (Example 1),and 15,000 cPs (Comparative Example 4), respectively, on a diffusionlayer by using a screen printing process, but also photographs of theelectrodes.

As is evident from FIG. 9, the electrode of Comparative Example 3 whichhad been screen-printed with catalyst ink having a viscosity of lessthan 100 cPs (75 cPs) showed the penetration of catalyst particles intothe diffusion layer due to the low viscosity of the catalyst ink. Thus,the electrode of Comparative Example 3 had catalyst loss and willprevent the diffusion of fuel. The electrode of Comparative Example 4which had been screen-printed with catalyst ink having a viscosity ofmore than 10,000 cPs (15,000 cPs) showed non-uniform coating due to thepoor flowability of the catalyst ink.

Meanwhile, the power densities of the membrane-electrode assembliesproduced in Example 1 and Comparative Examples 3 and 4 were measuredunder the following operation conditions of a single cell:

Anode catalyst: Pt/Ru black; cathode catalyst: Pt black; operationtemperature: 80° C.; amount of use of catalyst: 4 mg/cm²; fuel: 2MCH₃OH; 1000 cc/min of oxygen; and ambient pressure.

As shown in FIG. 10, the case of Comparative Example 3 (75 cPs) showed asharp reduction in performance, since catalyst particles plugged thepores of the diffusion layer upon screen printing so as to cause themass transfer resistance of fuel in a high-current region. Meanwhile,the case of Comparative Example 4 (15,000 cPs) showed low performance ina low-current region, since catalyst particles were coated at a loweramount than the desired loading amount due to the poor coating of thecatalyst ink.

4. Power Densities According to Coating Methods (Example 1 andComparative Example 6)

The power densities of the membrane-electrode assemblies produced inExample 1 and Comparative Example 6 were measured.

The power density measurements were carried out under the followingoperation conditions of a single cell:

Anode catalyst: Pt/Ru black; cathode catalyst: Pt black; operationtemperature: 80° C.; amount of use of catalyst: 4 mg/cm²; fuel: 2MCH₃OH; 1000 cc/min of oxygen; and ambient pressure.

In the case of Comparative Example 6 where the active layer was formedon carbon paper or carbon cloth used as the diffusion layer by spraycoating process and assembled with the electrolyte membrane by a hotpressing process, a significantly large amount of catalyst particlespenetrated into the diffusion layer by the spray coating process so asto make it impossible to participate in reaction, and acted asresistance to the diffusion of the fuel methanol solution. Accordingly,as shown in FIG. 12, it can be found that, in the case of ComparativeExample 6, the mass transport in a high-current region where reactionactively occurs to require the active supply of fuel reactants (methanolor hydrogen), is not smooth, resulting in a severe deterioration inperformance.

5. Power Densities According to Whether Catalyst Particles are Coatedwith Nafion or not (Examples 2 and 3)

The power densities of the membrane-electrode assemblies produced inExamples 2 and 3 were measured in a hydrogen fuel cell (PEMFC).

The power density measurements were carried out under the followingoperation conditions of a single cell:

Anode catalyst: Pt/C; cathode catalyst: Pt/C; operation temperature: 70°C.; amount of use of catalyst: 0.4 mg/cm²; fuel: H₂; and ambientpressure of air.

FIG. 15 shows the comparison between Example 2 where the catalystparticles were coated with electrolyte (Nafion) and Example 3 where thecatalyst particles were not coated with the electrolyte (Nafion). Asshown in FIG. 15, the electrode fabricated using the electrolyte(Nafion)-coated catalyst particles had a relatively low mass transportresistance, indicating superiority in performance.

In the case of Example 2, an electrode structure was formed in which theelectrolyte (Nafion)-coated catalyst particles were coated on thediffusion layer so as to maintain the pores among catalyst particles atthe maximum as shown in the left figure of FIG. 14. The maintained poresprovide an improvement in the electrode performance by forming a passagethrough which fuel can be smoothly supplied to the active section of thecatalysts.

1. A membrane-electrode assembly comprising: electrode consisting of aanode comprising a gas diffusion layer and a catalystmaterial-containing active layer, and an cathode comprising a diffusionlayer and a catalyst material-containing active layer; and anelectrolyte membrane interposed between the anode and the cathode andcomprising a catalyst material-containing active layer at one or bothsides, the electrodes being hot-pressed to the electrolyte membrane,wherein the viscosity of the active layer in coating the active layer onthe gas diffusion layer is in a range of 100 to 10,000 cPs.
 2. Themembrane-electrode assembly of claim 1, wherein the viscosity of theactive layer in coating the active layer on the gas diffusion layer isin a range of 1,000 to 10,000 cPs.
 3. The membrane-electrode assembly ofclaim 1, wherein the catalyst particles forming the active layer ofelectrode are coated with an electrolyte.
 4. The membrane-electrodeassembly of claim 1, wherein the catalyst coated on a anode side-surfaceof the electrolyte membrane is the same as the catalyst of the activelayer in the anode, and the catalyst coated on an cathode side-surfaceof the electrolyte membrane is the same as the catalyst of the activelayer in the cathode.
 5. The membrane-electrode assembly of claim 1,wherein the active layer on the gas diffusion layer is coated on the gasdiffusion layer by a curtain coating process.
 6. The membrane-electrodeassembly of claim 1, wherein the active layer on the electrolytemembrane is coated on the electrolyte membrane by a spray coatingprocess at viscosity of less than 10 cPs.
 7. The membrane-electrodeassembly of claim 1, wherein the amount of the active layer formed onthe electrolyte membrane is 1-100% by weight based on the weight of theactive layer formed on the gas diffusion layer.
 8. A method forproducing a membrane-electrode assembly as set forth in claim 1, themethod comprising the steps of: (a) forming a catalystmaterial-containing active layer on the surface of a electrolytemembrane; (b) forming a catalyst material-containing active layer on thesurface of a gas diffusion layer; and (c) hot-pressing the gas diffusionlayer to the electrolyte membrane, wherein the viscosity of the activelayer, which is applied on the gas diffusion layer at the step (b), iscontrolled in a range of 100 to 10,000 cPs.
 9. The method of claim 8,wherein, at the step (a), catalyst ink fed by a gas pressure method iscoated on the dried electrolyte membrane by a spray process.
 10. Themethod of claim 8, wherein, wherein the viscosity of the active layer,which is applied on the electrolyte membrane at the step (a), is lessthan 10 cPs.
 11. The method of claim 9, wherein, at the step (a), theelectrolyte membrane is maintained in a dried state by a thermal dryer.12. The method of claim 8, wherein the step (b) is performed by coatingthe catalyst with electrolyte powder, mixing the coated catalyst powderwith a solvent so as to prepare catalyst ink, and coating the catalystink on the gas diffusion layer so as to form the active layer.
 13. Themethod of claim 8, wherein the step (a) is carried out at an operationtemperature of 20-100° C.
 14. The method of claim 8, wherein the step(c) is carried out at an operation temperature of 50-200° C. under apressure of 5-100 kg/cm².
 15. The method of claim 8, wherein the step(b) further comprises performing a dry coating process to the gasdiffusion layer.
 16. A membrane-electrode assembly comprising anelectrolyte (ionomer)-coated catalyst particles at a catalytic activelayer.